Process for making high quality hotworked products from aluminum base alloy powders



United States Patent 3,544,392 PROCESS FOR MAKING HIGH QUALITY HOT-WORKED PRODUCTS FROM ALUMINUM BASE ALLOY POWDERS John P. Lyle, Jr., NewKensington, Pa., and Raymond J. Towner, Lima, Ohio, assignors toAluminum Company of America, Pittsburgh, Pa., a corporation ofPennsylvania No Drawing. Filed Apr. 8, 1968, Ser. No. 719,742. Int. Cl.C2215 1/04; C2111 9/00 US. Cl. 148-115 14 Claims ABSTRACT OF THEDISCLOSURE Production of articles of improved ultrasonic quality made ofaluminous metal powder by compacting powder to from 60% to less than100% of theoretical density, degassing the resulting compact by heatingwithout sintering at 800 to 1125 F. in a substantially dry inertatmosphere substantially free of hydrogen and then substantiallyimmediately compressing the heated compact to substantially 100% of itstheoretical density before hot working.

BACKGROUND OF THE INVENTION This invention involves production ofaluminous metal powder alloy articles. More particularly, it involves amethod for use of powder metallurgy techniques to produce from aluminousmetal powder a substantially degassed shaped article of improvedultrasonic quality. The term aluminous metal as used herein refers toboth aluminum and the alloys in which this element predominates. It alsois intended to cover mixtures of aluminous powdered metals in whichaluminum or aluminum base alloy powder is the major component. Theinvention herein described was made in the course of or under a contractor subcontract thereunder with the Department of the Army.

It is known, for example, as taught in Ennor and Lyle US. Pat.2,809,891, to compact and heat aluminous metal powder in a protectiveatmosphere for A to 36 hours at an elevated temperature of between 840and below the 1220 F. liquidus temperature of aluminum to a density ofless than 100% of theoretical, to heat the resulting compact to a hotworking temperature and then to hot work the final compact, for example,by transferring to an extrusion die. Theoretical density is the densityof a completely consolidated hot worked article which has no substantialporosity. Towner and Lyle, in US. Pats. Nos. 2,966,731- and 2,963,780,disclose initially heating aluminum base alloy powder at 700900 F.,compacting the powder at 200 to 150,000 psi. for from a minute toseveral hours, heating the compacted powder at 700-900 F. (except for analloy containing 3% chromium, molybdenum, tungsten, titanium, zirconium,or manganese, which may be heated to 1150 F.), and then compressing theheated powder and finally hot working it by extruding, rolling, orforging. MacDonald and Ransley, in Symposium on Powder Metallurgy(England, 1954) at p. 245 of a paper entitled, Preparation of HighModulus Aluminum Alloys by Powder Metallurgy, disclose sintering anickel aluminum alloy powder in highly purified hydrogen at about 1137"F. for an hour and then hot pressing the sintered powder to close to100% of its theoretical density. Blisters, discontinuities or voids aresometimes noticeable in articles produced by such fabricatingprocedures, especially after the heating which is used in the extrusionor shaping step or in subsequent solution heat treatment, artificialaging or annealing in specific instances, thus making them lessacceptable under current requirements for substantial freedom fromdefects 3,544,392 Patented Dec. 1, 1970 ice in certain airframecomponents which must undergo conditions of high stress.

OUTLINE OF THE INVENTION It is accordingly an object of this inventionto provide a process for preparation of aluminous metal powder articlesof improved structural soundness. It is a further objectof thisinvention to provide a method for treatment of aluminous metal powdersuch that the articles produced are substantially free from defects suchas blisters, discontinuities and voids. Further objects of thisinvention will be apparent from the description and claims which follow.

Our invention is predicated upon the discovery that when aluminous metalpowder is compacted to above 60% but less than 100% of theoreticaldensity, preferably 65-90%, and then heated without sintering in aflowing, substantially dry inert atmosphere which is substantially freeof hydrogen at from about 800 to about 1125 F. and then substantiallyimmediately compressed to above of theoretical density, that issubstantially of theoretical density and hot worked into a shapedarticle (for example, by hot pressing and then extruding), the presenceof blisters, discontinuities and voids in the hot worked product can beminimized or avoided completely, and an improved ultrasonic ratingimparted to the article. By improved ultrasonic rating or quality wemean one which is at least as high as the Class A standard (we use A+herein to indicate this improved ultrasonic rating) of the Society forNondestructive Testing (SNT) Airframe Subcommittee Report No. 1(Revised), Feb. 28, 1964, pp. 366, 367 and 368, entitled, RecommendedUltrasonic Acceptance Standards for Airframe Aluminum Alloy Plate,Extrusion and Forgings. This is a standard ultrasonic inspection testfor checking against discontinuities which are not generally visible tothe naked eye. Some evidence of improved ultrasonic rating is also to befound by visual inspection for absence of blistering and cracking. Wehave found that aluminous metal powder articles produced by our methodeven after the solution heat treatment and artificial aging required forsome end uses and as described in Sprowls et al. US. Pat. No. 3,198,676retain the improved or A+ ultrasonic rating or quality.

In stating that the inert atmosphere used in the heating step must besubstantially dry, we mean an inert atmosphere which has a dew point of-40 F. or lower.

According to our process the heating step which follows the initialcompacting step may take from about 1 to about 36 hours. The preferredtime is from 12-24 hours. In the heating step it is essential that thetemperature be high enough to be of help in the compression step whichfollows substantially immediately but not high enough to result insintering. Thus, it should be above about 800 F. but below the fusionpoint of aluminum and not higher than about 1125 F.

While we do not wish to be bound by any theory, it appears that mostaluminous metal powder contains at least some hydrogen (possibly formedfrom water present) and water entrapped therein which may be detrimentalto the preferred substantially blister-free, void-free anddiscontinuity-free condition. Furthermore, the heating and compactingprocedures heretofore known apparently tend to cause additionaloxidation inside any pores which may exist, thus possibly helping tokeep the aluminous metal powder articles from achieving the ultrasonicrating sometimes required. Therefore, as mentioned above, in our processwe use a substantially dry and substantially hydrogen-free inertatmosphere in the heating step which substantially immediately precedesour compressing step. The inert atmosphere must also be flowing in orderto carry away hydrogen and water and other gases or vapors evolvedduring the heating. Examples of suitable substantially hydrogen-free,flowing, substantially dry inert atmospheres includes a vacuum,nitrogen, helium and argon. One reason for the requirement according toour invention that the inert atmosphere be substantially hydrogen-freeis that hydrogen is one of the gases believed to cause some of thedifficulties sometimes encountered by way of occasional blisters,discontinuities and voids in compacted aluminous metal powder andarticles formed therefrom.

Controlling the extent of compacting of the powder to from about 60 toless than 100%, preferably 65-90, of theoretical density, in the firstcompacting step, and compressing to substantially 100% of theoreticaldensity substantially immediately after the intermediate heating stepfurther assist in eliminating undesirable voids and defects. Bestresults were obtained when compacting in the first step at 72-87% oftheoretical density. The intermediate heating step between the initialcompacting step and the final compression prior to hot working alsopermits bringing of the compacted powder to the temperature desired forthe hot pressing step.

It may be desirable to use a heated compression chamber during the finalcompression step prior to hot working. The temperature during thiscompression and in the subsequent hot working generally ranges fromaround 500-900 F., preferably from about 600 to about 700 F. Ordinarilyat least 40,000 p.s.i. pressure is desirable to obtain substantially100% of theoretical density desired with absence of any substantialamount of porosity. The hot working may be by extrusion, forging,rolling or the like, our preferred method being extrusion.

The aluminous metal powders which are adapted to be compacted,heated,'compressed and hot worked according to our process may be of themilled or of the atomized type. Powder prepared by air atomization andcollection in air is preferred to eliminate as much undesirable moistureas possible. Atomization and collection procedures such as thosedescribed in the aforementioned Towner and Lyle U.S. Pats. Nos.2,966,731-5 and 2,963,- 780 may be used. A wide variation in particlesize is permissible, but the particles are preferably not larger thanwill pass through a 100-mesh screen. It is preferred to utilize powdersof a very fine mesh size, preferably of from 5 to 60 microns mass mediandiameter (MMD) as measured by a Sharples micromerograph. Best resultsare obtained when the particles are from 18 to 54 microns in size.

The metal powder used in our process may consist of aluminum ofcommercial purity, for example, 99%, up to the highest purityobtainable, or it may consist of particles of aluminum base alloys, orit may be made up of a mixture of particles of aluminum and the desiredalloying ele ments. The elements which may be thus associated withaluminum in either alloyed or elemental form are those commonly employedin the aluminum base alloy art, such as, copper, magnesium, silicon,zinc, manganese, iron and certain high melting point elements. They maybe present in the quantity normally used in the solid metal alloys, forexample, up to 10% copper, up to silicon, up to 13% zinc, up to 10%magnesium. Such high melting point elements as nickel, chromium,

titanium, boron, and zirconium may also be employed in amounts up to0.75% each. Especially when the aluminous metal powder is an alloy, asindicated above, it may be desirable to subject the compact orsubsequent product to a specific thermal treatment with or without asubsequent precipitation hardening treatment.

For our initial compacting step any temperature below which meltingbegins is generally satisfactory. The pressure range is not critical, itbeing suflicient to use a pressure which will give the desired amount ofcompacting to eliminate too much porosity and yet to provide a compactof the desired strength which will withstand the conditions encounteredin the subsequent heating and hot working steps. Room temperature (GS-90F.) is usually satisfactory. Higher temperatures, of course, requirelower pressures to obtain the same desired density. As indicated above,the heating temperature for the intermediate heating step which followsthe initial compacting step must be below that at which sintering of thepowder occurs, and the temperature during the subsequent compression andhot working steps must also be below the fusion point.

The table which follows shows the characteristic ultrasonic rating aswell as visual inspection appearance for representative aluminous metalpowder articles prepared according to our invention. Air atomized andair collected powder was used. A flowing atmosphere of argon (10 cu.ft./min.) substantially free of hydrogen and moisture was used duringthe intermediate heating step which preceded the hot working. For ourinitial compacting step, we used a compression chamber which had asmooth cylindrical portion at one end and an outward taper on the otherend. A hydraulically operated dummy block moved in the cylindricalportion toward a blind die during a compacting stroke, stopping when theface of the dummy block reached the point where the taper began. Theblind die was removed and force applied to the dummy block to push thecompact out of the compacting die. The cylinder used was about sixinches in diameter, and the tapered wall portion had about a 1 taper.The cylinder was lubricated with butyl stearate before the powder wascharged into it.

Heating for the intermediate heating step prior to the final compressionstep was at the temperatures and for the times shown in the table. Inthe compression step which follows the intermediate heating step andpriorto the hot working step, the samples were pressed against a blinddie in an extrusion cylinder at substantially full press capacity ofabout 93,000 p.s.i. and at temperatures below the fusion point. For thehot working step, the blind die was replaced with a 2"-diameterextrusion die and the article then extruded at a temperature of about650 F.

The test procedure used for determining the ultrasonic rating of thesamples in the table which follows (except for Prior Art Samples 1 and2, for which a visual rating was obtained) was substantially the same asthe SNT method referred to above except that a combination of referenceblock and instrument settings was used which was capable of detectingmuch smaller discontinuities than the SNT Class A method, the ratio ofdiscontinuity sizes being about 1 to 8.

TABLE Percent of theoretical Pressure density to Temperature in appliedwhich uring initial compacted heating step Time of heating Ifowdercompacting in initial prior to step prior to size p.s.i. step,compacting compressing compressing Sample MlViD thousands step step, F.step, hours Prior art 1 100mesh. 93 900 1 Prior art 2 100 mesh-- 93 l,000 1 1 36 76 1, 100 13. 5 87 900 8. 5 92 860 11. 25 102 84 900 18. 5110 77 860 12. 25 83 S50 19 See footnotes at end of table.

TABLE-Continued Blistering (visual) Cracking (visual) Ultrasonic ratingBefore SHT 9 After SET 9 and Sample Before SHT 9 After SHT and AA AAPrior art 1 Poor (visual) Poor (visual) Prior art 2 Poor (visual) Poor(visual).

A+.- A A+--- A+ 13?; art sample 1 contained in percent by weight 6.1 Zn,2.9 Mg, 1.6 Cu, 0.24 Cr, 0.05 Ti.

2 gig art sample 2 contained in percent by weight 9.8 Zn, 3.5 Mg., 1.0Cu, 0.04 Ti, 0.46 Mn, 0. 2 s.

3 Sample 1 contained in percent by weight 5.5 Zn, 2.5 Mg, 1.6 Cu, 0.25Cr.

4 Sample 2 contained in percent by wright 10.9 Zn, 4.9 Mg, 2.0 Cu, 1.7Mn.

6 Sample 3 contained in percent by weight 10.3 Zn, 2.9 Mg, 2.1 Cu, 1.8Mn, 0.14 Cr. 6 Sample 4 contained in percent by weight 7.8 Zn, 3.5 Mg,1.6 Cu, 0.6 Mn, 0.5 Cr, 0.5 each Fe, Co, Mo,W, 0.4 each Ti, V, 0.6 eachZr, Ni.

7 Sample 5 contained in percent by weight 9.8 Zn, 4.2 M 4.4 5 Sample 6containedin percent by weight 2.9 Zn,l4.6 Mg, 2.4 Cu. 9 Solution heattreatment 2 hours at 860 F. Artificial aging or age hardening 24 hoursat 250 F.

Cu, 2.1 Mn, 1.6 Fe, 4.6 Ni.

11 Prior art samples 1 and 2 were air atomized, air collected, screened,compacted in a 6.38 diameter cylinder at 93,000 p.s.i. and cylindertemperature of 650 F., ejected, then heated in air 1 hour at 900-1900F., cooled and scalped to 6.25" diameter, reheated and extruded to 2diameter rods.

12 Samples l-6 contained less than 1% by weight A120 From the foregoingdata it is readily apparent that, by compacting aluminous metal powderto from about 60 to less than 100% of theoretical density, degassing thecompact by heating in a substantinally dry, flowing inert atmospheresubstantially free of hydrogen and then substantially immediatelycompressing the heated compact to substantially 100% of theoreticaldensity prior to hot Working, we have substantially elminated blisters,discontinuities and voids caused by entrapped hydrogen and/ or othergases, and enable production of aluminous metal powder articles whichhave an improved ultrasonic rating.

While the invention has been described in terms of preferredembodiments, the claims appended hereto are intended to encompass allembodiments which fall within the spirit of the invention.

Having thus described our invention and certain embodiments thereof, weclaim:

1. A process for the production of aluminous metal powder articles whichcomprises (1) compacting aluminous metal powder to from about 60 to lessthan 100 of theoretical density,

(2) heating the resulting compact below the fusion point of the metal ina flowing inert atmosphere substantially free of hydrogen at from about800 F. to about 1125 F, thereby minimizing formation of blisters,discontinuities and voids during subsequent heating,

(3) then compressing said compact to above 90% of theoretical density,and

(4) hot working said compact into a shaped article having an improvedultrasonic rating.

2. The process of claim 1 wherein the compacting of the powder is to 65to 90% of theoretical density.

3. The process of claim 1 wherein the aluminous metal powder comprisesair atomized and air collected powder screened to a particle size offrom 5 to 60 microns mass median diameter, and the heating step isperformed in an argon atmosphere.

4. The process of claim 1 wherein the heating is for from about 12 toabout 24 hours and the hot working is followed by solution heattreating.

5. The process of claim 1 wherein the hot working is followed bysolution heat treating and artificial aging.

6. The process of claim 1 wherein the hot working comprises extrusion.

7. The process of claim 1 wherein the hot working is followed bysolution heat treatment.

8. The process of claim 1 wherein the hot working is followed bysolution heat treatment and then artificial aging.

9. The process of claim 1 wherein the aluminous metal powder comprisesair atomized and air collected powder screened to a particle size of18-54 microns mass median diameter and the hot working is followed bysolution heat treatment and then artificial aging.

10. The process of claim 1 wherein the aluminous metal powder comprisesan aluminum base alloy containing zinc and magnesium.

11. The process of claim 1 wherein the aluminous metal powder comprisesan aluminum base alloy containing zinc, magnesium and copper.

12. A process for preparation of an aluminous metal powder shapedarticle which comprises the following steps in successive order:

(1) atomizing a melt of an aluminous metal in air,

(2) collecting the resulting aluminous metal powder 1n air,

(3) screening said powder to a particle size of 18-54 microns massmedian diameter,

(4) compacting said powder to from 65 theoretical density,

(5) heating the compacted powder below the fusion point of the metal at800-112S F. in a flowing inert 7 atmosphere substantially free ofhydrogen for 8-24 References Cited hours, (6) then hot pressing theheated compacted powder to 7 UNITED STATES PATENTS above 90% oftheoretical density, and then 3,226,267 12/1965 Foerster 148-l1.5 (7)extruding the resulting hot pressed, heated com- 3,462,248 8/1969Roberts t a1. 75- 138 pacted powder, thereby forming a shaped articlewhich has an improved ultrasonic rating when in L. DEWAYNE RU'ILEDGE,Primary Examiner solution heat treated condition. 13. The process ofclaim 12 wherein the aluminous STALLARD Asslstant Exammer metal powdercomprises an aluminum base alloy contain- 10 U 8 Cl X R ing zinc andmagnesium. 148 12 7 14. The process of claim 12 wherein the aluminousmetal powder comprises an aluminum base alloy containing zinc, magnesiumand copper.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,544,392 Dated December 1, 1970 Inventor(s)-.I9hn P. Lyle, Jr. andRaymond J. Towner II in curtifh-(l that error appears in theabove-identified patent and that maid Letters Patent are herebycorrected as shown below:

Col. 5, Line 60, change "60" to "607." and "100" to "10079-- C01. 6,Line 72, change "90" to "907,--

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